Process for the production of propylene from olefinic streams

ABSTRACT

Process for the production of propylene starting from mixtures of hydrocarbons, prevalently olefins, the above hydrocarbons having a boiling point ranging from −15° C. to +80° C., preferably from −12° C. to +60° C., which comprises putting the above mixture of hydrocarbons in contact, under cracking conditions, with a large-pore zeolite having a molar ratio Silica/Alumina lower than 200, preferably ranging from 50 to 150.

[0001] The present invention relates to a process for the production ofpropylene from prevalently olefinic hydrocarbon streams.

[0002] More specifically, the present invention relates to a selectivecracking process for the production of propylene starting from mixturesof hydrocarbons, prevalently olefins, the above hydrocarbons having aboiling point ranging from −15° C. to +80° C., preferably from −12° C.to +60° C.

[0003] A typical example of these fractions are essentially C₄-C₆fractions coming from steam cracking and catalytic cracking, having anolefin content of at least 40% by weight, usually at least 70% byweight.

[0004] Propylene is one of the most important chemical products from thepoint of view of demand and production volume and is mainly used in theproduction of polymers. The main propylene source is the steam crackingprocess, in which hydrocarbon charges with a high paraffin content arethermally treated in the presence of vapour. The main products of steamcracking are propylene and ethylene, which leave the process in a ratioof about 0.5. As the market request for propylene has become greater inthe last few years with respect to that of ethylene and owing to thefact that the propylene/ethylene ratio cannot be significantly varied,it has become necessary to increase the production of propylene usingalternative methods. In fact, in 1999 there was a considerable deficitof propylene in Western Europe, with importations of this productamounting to 195,000 tons. The production of large quantities ofpropylene by means of processes which appropriately treat steam crackingby-products can allow variations in the overall propylene/ethyleneratio, thus meeting market demands. The possibility of having flexibleprocesses allowing a certain flexibility in the propylene/ethylene ratiowould therefore provide great economic advantages.

[0005] An interesting possibility consists in a selective catalyticcracking process which converts C₄-C₅ fractions to propylene. Thefractions can derive from steam cracking but it is also possible toextend the process to other similar streams coming for example from FCC(Fluid Catalytic Cracking). The charges can also derive from the abovefractions after extraction and/or enrichment in olefins.

[0006] The use of solid acid catalysts, among which amorphoussilico-aluminas and in particular zeolites, in the cracking reaction ofhydrocarbons, is known in literature (see for example J. Scherzer, Cata.Rev.—Sci. Eng., 31(3), 215-354, 1989).

[0007] The most important application of these materials in crackingreactions, from an industrial point of view, is that called FCC (FluidCatalytic Cracking) whose purpose, starting from heavy charges such asvacuum gas oils, is to produce lighter hydrocarbon cuts, particularlywithin the boiling range of gasolines. The catalysts currently used inthis process are Y-type zeolites (IUPAC abbreviation: FAU) containingvarious additives.

[0008] A different type of cracking, owing to the charges used and typeof products to be obtained, is called “selective cracking”. The purposeof “selective cracking” is to produce light olefins, such as ethyleneand propylene, starting from C₄-C₆ hydrocarbon fractions and thereforealready light, if compared to a vacuum gas oil. The advantage of thisprocess consists in transforming low quality hydrocarbon fractions,difficult to distribute on the market, to olefins having a higher addedvalue.

[0009] Various zeolitic materials active in “selective cracking”reactions, are described in literature. For example, EP-A-109,059 andEP-A-109,060 describe the use of ZSM-5 zeolite (IUPAC abbreviation: MFI)for selective cracking reactions. These documents demonstrate that thebest catalytic performances, referring to yields to propylene andethylene, are obtained when the SiO₂/Al₂O₃ ratio of the zeolite is high.More specifically, EP-A-109,059 claims, for MFI-type zeolites (ZSM-5)SiO₂/Al₂O₃ ratios lower than or equal to 300 (mol/mol), preferablybetween 25 and 220, whereas EP-A-109,060 discloses SiO₂/Al₂O₃ ratioshigher than or equal to 350 (mol/mol).

[0010] WO 99/57226 describes a method for converting hydrocarboncharges, with a boiling point within the naphtha range, to propylene inthe presence of medium-pore zeolites having an SiO₂/Al₂O₃ ratio greaterthan 200 (mol/mol). The above document provides two experimentalexamples: in the first, three medium-pore zeolites ZSM-48(SiO₂/Al₂O₃>1500), ZSM-22 (SiO₂/Al₂O₃>1500) and ZSM-5 (SiO₂/Al₂O₃=55)are compared. It is shown that the selectivity to propylene for thefirst two catalysts is higher with respect to ZSM-5. In the secondexample, two ZSM-22 zeolites having a different SiO₂/Al₂O₃ ratio (>1500and 120) are compared. It is shown that the one with the greaterSiO₂/Al₂O₃ ratio has the higher selectivity to propylene.

[0011] Finally, WO 99/29805 describes a process for producing propylenestarting from C₄ and higher olefinic streams, in the presence of MFIzeolite (ZSM-5) having an SiO₂/Al₂O₃ ratio of at least 180 (mol/mol).

[0012] Experts in the field, however, still feel the necessity for usingmaterials suitable for obtaining greater conversions and at the sametime having a higher stability of the catalytic activity over a periodof time. An extremely important problem, in fact, which is not takenmuch into consideration in literature, consists in the poor stability ofthe catalytic material over a period of time.

[0013] A process has now been found, which uses materials capable ofimproving the yield to propylene and that also have the great advantageof maintaining the catalytic performances practically constant over aperiod of time.

[0014] In accordance with this, the present invention relates to aprocess for the production of propylene starting from mixtures ofhydrocarbons, prevalently olefins, the above hydrocarbons having aboiling point ranging from −15° C. to +80° C., preferably from −12° C.to +60° C., which comprises putting the above mixture of hydrocarbons incontact, under cracking conditions, with a large-pore zeolite having amolar ratio Silica/Alumina lower than 200, preferably ranging from 50 to150.

[0015] The hydrocarbon mixtures essentially consist of hydrocarbons,both olefins and paraffins, having a boiling point ranging from −15° C.to +80° C., preferably from −12° C. to +60° C. Typical examples ofhydrocarbons forming the above hydrocarbon mixtures are 1-butene,trans-2-butene, cis-2-butene, n-butane, isobutane, propane, pentane,isopentane, 1-pentene, 2-pentene, n-hexane, 1-hexene, 2-hexene. Thehydrocarbon mixtures comprise from 30% to 100% by weight of olefins,preferably from 40% to 85% by weight. The paraffins contained in thehydrocarbon mixtures range from 5% to 65% by weight, preferably from 10%to 50% by weight, even more preferably from 20% to 45% by weight.

[0016] The term “cracking conditions” refers to a temperature at whichthe contact between the hydrocarbon mixtures and catalyst takes place,ranging from 400° C. to 750° C., preferably from 450° C. to 700° C.,even more preferably from 500° C. to 650° C.

[0017] The process of the present invention is preferably carried out ata weight hourly space velocity (WHSV) ranging from 0.1 h⁻¹ to 1,000 h⁻¹,more preferably from 0.5 h⁻¹ to 100 h⁻¹, even more preferably from 0.8h⁻¹ to 50 h⁻¹.

[0018] The pressure in the contact zone between catalyst and hydrocarbonmixtures ranges from 0.1 to 30 absolute atm., preferably from 1 to 3absolute atm., more preferably about 1 absolute atm.

[0019] The process of the present invention can be carried out using anyreactor solution, for example, fixed bed, moving bed, a “riser” reactoror a fluid bed, preferably fixed bed.

[0020] The catalyst which can be used in the process of the presentinvention is a large-pore zeolite having a molar ratio Silica/Aluminalower than 200, preferably ranging from 50 to 150. The term “large-porezeolite” refers (see N. Y. Chen and T. F. Degnan, Chemcial EngineeringProgress, February 1988, 32-41) to a zeolite have a lattice consistingof 12 tetrahedrons. The above zeolite has a molar ratio Silica/Aluminalower than 200, preferably ranging from 50 to 150. In the preferredembodiment, the zeolite is ZSM-12 (IUPAC abbreviation: MTW), having amolar ratio Silica/Alumina lower than 200, preferably ranging from 50 to150. The preparation of this zeolite is well known to experts in thefield.

[0021] The zeolite can be used as such or mixed with inert products, inthe form of granules or pellets.

[0022] Contrary to what is specified in scientific and patentliterature, the ZSM-12 material has the best catalytic performances atSiO₂/Al₂O₃ ratios<200 (mol/mol). The best catalytic performances referto both yields to propylene and stability (duration) of the catalystover a period of time.

[0023] The following examples are provided for a better understanding ofthe present invention.

EXAMPLES

[0024] The catalytic testing experiments were carried out in acontinuous laboratory plant, with a fixed bed tubular reactorconfiguration. The reaction products were characterized with agaschromatograph model HP 5890 equipped with a “PONA” capillary column.

[0025] The synthetic mixture of C₄ hydrocarbons indicated in Table 1 wasused for the experimental tests. This mixture has a similar compositionto the stream called “Refined III” deriving from steam cracking. TABLE 1Mixture used in the catalytic tests Feeding Hydrocarbon (weight %)1-butene / Trans-2-butene 52.35 Cis-2-butene 24.86 n-butane 22.61Iso-butane 0.18 Sum of olefins 77.21

[0026] The weight quantities of hydrogen, methane, ethane, ethylene,propane, propylene, n-butane, isobutane, 1-butene and isobutene,cis-2-butene, trans-2-butene, butadiene and a fraction of heavierproducts called C₅ ⁺, were determined in the gaseous reaction products.

[0027] The catalyst was charged in a quantity varying from 2 to 10 g, ingranules of 20-40 mesh or in pellets of 2-4 mm, mixed with corindone(inert product), in a weight ratio of 1:1.

Example 1 Synthesis of ZSM-12 (SiO₂/Al₂O₃=100 mol/mol)

[0028] 2.4 g of sodium aluminate with a content of Al₂I₃ equal to 56%are added to an aqueous solution of tetramethylammonium hydroxide at35%. The solution thus obtained is poured, under stirring, into 200 g ofcolloidal silica Ludox HS 40.

[0029] A limpid, homogeneous gel is obtained, which is poured into anAISI316 steel autoclave, equipped with an anchor stirrer. The gel isleft to crystallize under hydrothermal conditions at 165° C. for 90hours.

[0030] After cooling the autoclave, the solid obtained is separated fromthe mother liquor and washed with demineralized water until the washingwater has a pH of less than 9.

[0031] The solid obtained is calcined at 550° C. in a stream of air for5 hours.

[0032] The solid thus obtained is subjected to ionic exchange by meansof suspension in an aqueous solution of ammonium acetate. The ammoniumion is present in excess with respect to the nominal aluminum present inthe solid. After filtration and washing of the solid, the wholeoperation (exchange and washing) is repeated.

[0033] The solid obtained is calcined at 550° C. in a stream of air for5 hours.

[0034] The zeolitic solid is thus obtained in its acid form which, uponXRD analysis reveals the presence of the sole crystalline phase of theZSM-12 type (MTW). Chemical analysis shows a content of residual sodiumof less than 50 ppm and a molar ratio SiO₂/Al₂O₃=100.

Comparative Example 2 Synthesis of ZSM-12 (SiO₂/Al₂O₃=250 mol/mol)

[0035] 0.97 g of sodium aluminate with a content of Al₂O₃ equal to 56%are added to an aqueous solution of tetramethylammonium hydroxide at35%. The solution thus obtained is poured, under stirring, into 200 g ofcolloidal silica Ludox HS 40.

[0036] A limpid, homogeneous gel is obtained, which is poured into anAISI316 steel autoclave, equipped with an anchor stirrer. The gel isleft to crystallize under hydrothermal conditions at 165° C. for 90hours.

[0037] After cooling the autoclave, the solid obtained is separated fromthe mother liquor and washed with demineralized water until the washingwater has a pH of less than 9.

[0038] The solid obtained is calcined at 550° C. in a stream of air for5 hours.

[0039] The solid thus obtained is subjected to ionic exchange by meansof suspension in an aqueous solution of ammonium acetate. The ammoniumion is present in excess with respect to the nominal aluminum present inthe solid. After filtration and washing of the solid, the wholeoperation (exchange and washing) is repeated.

[0040] The solid obtained is calcined at 550° C. in a stream of air for5 hours.

[0041] The zeolitic solid is thus obtained in its acid form which, uponXRD analysis reveals the presence of the sole crystalline phase of theMTW type. Chemical analysis shows a content of residual sodium of lessthan 50 ppm and a molar ratio SiO₂/Al₂O₃=250.

Example 3 Catalytic test with ZSM-12 (SiO₂/Al₂O₃=100 mol/mol)

[0042] The catalytic testing of ZSM-12 zeolite having a molar ratioSiO₂/Al₂O₃=100 (example 1), was carried out using the equipmentdescribed above and under the following operating conditions:

[0043] Reaction T=500° C.;

[0044] Total pressure=1 bar;

[0045] WHSV_(total)=1 h⁻¹;

[0046] Feeding see Table 1.

[0047] The WHSV is defined as a ratio between the hourly weightflow-rate (g/h) of the mixture in the feeding divided by the weight ofthe catalyst (g). From a dimensional point of view it is h⁻¹.

[0048]FIG. 1 indicates the two curves relating to total conversion andselectivity to propylene, obtained with the catalyst ZSM-12 having amolar ratio SiO₂/Al₂O₃=100, in relation to the time on stream (tos).

[0049] The total conversion is defined as follows:

Tot. conv. %=[(C₄ at the reactor inlet)−(C₄ at the reactor outlet)]/(C₄at the reactor inlet)·100.

[0050] In this way the C₄ fraction is not divided into olefins orparaffins but is considered altogether as a potential reagent.

[0051] The selectivity to propylene is calculated as:

selectivity to propylene%=(yield to propylene)/(total conversion)·100.

[0052] The yield to propylene is experimentally obtained bygaschromatographic analysis.

[0053] In addition to the high conversion and selectivity values, theunexpected stability of this material over a period of time is extremelyimportant. In fact, it can be noted from the graph of FIG. 1 that nocatalytic deterioration phenomena are present until at least 140 h oftos.

[0054] This stability over a period time makes the material particularlysuitable for use in simple reactor conditions such as fixed beds.

[0055] More complicated solutions, however, such asfluidized/transported beds can obviously also be used.

[0056] Table 2 indicates, for illustrative purposes, the selectivity ofthe different components forming the product at the outlet of the plant.Among olefins of interest, ethylene is also present (7.88%). C₅ ⁺refersto the liquid fraction, at atmospheric pressure and room temperature, ofthe product leaving the plant. Owing to the high number of hydrocarbonspresent in the C₅ ⁺fraction, Table 3 indicates the composition of thisfraction subdivided by group of compounds. As the composition of theproducts depends on the operating conditions, Table 3 specifies twodistributions obtained at two different reaction temperatures. TABLE 2Selectivity after 146 hours at a conversion of 55% Selectivity Product(weight %) Hydrogen 0.38 Methane 0.8 Ethylene 7.88 Ethane 0.78 Propylene39.1 Propane 5.54 C₅ ⁺ 45.52 Total 100.00

[0057] TABLE 3 Composition of the C₅ ⁺ liquid fraction obtained withZSM-12 (SiO₂/Al₂O₃ = 100 mol/mol) in weight % Reaction C₅ ⁺ T BTXNAPHTHALENES (non aromatics) OTHERS (° C.) % % % % 500 40 8 17.5 34.5500 55 7 5.5 32.5

[0058] The term BTX refers to benzene, toluene and xylenes. The headingNAPHTHALENES comprises all hydrocarbons, variously substituted, of thenaphthaline family. The term C₅ ⁺(non aromatics) refers to non aromatichydrocarbons, saturated and mono-unsaturated, containing 5-8 carbonatoms. The term OTHERS comprises those products for which it was notpossible to effect a gaschromatographic characterization

[0059] It can be seen how among the by-products, there are largequantities of easily exploitable products such as BTX.

Comparative Example 4 ZSM-12 (SiO₂/Al₂O₃=250 mol/mol)

[0060] The catalytic testing of ZSM-12 zeolite having a molar ratioSiO₂/Al₂O₃=250, whose synthesis is described in example 2, was effectedusing the equipment described above and under the exact operatingconditions specified in example 3.

[0061]FIG. 2 indicates the two curves relating to total conversion andselectivity to propylene, obtained with this zeolite in relation to thetime on stream (tos). The conversion and selectivity to propylene aredefined as in example 3.

[0062] Contrary to what is specified in literature, the catalyticperformances of ZSM-12 with a ratio SiO₂/Al₂O₃=250, are lower both interms of yield (product of selectivity and conversion) and duration,with respect to the zeolite having a greater content of Al₂O₃.

[0063] It can be observed, in fact, from the graph of FIG. 2 how,already after 25 hours of tos, evident catalytic deterioration phenomenaare present.

[0064] Table 5 indicates, for illustrative purposes, the selectivity ofthe various components forming the product at the plant outlet. Table 6,on the other hand, indicates the composition of the C₅ ⁺liquid fractionsubdivided by group of compounds. TABLE 5 Selectivity after 24 hours ata conversion of 52% Selectivity Product (weight %) Hydrogen 0.4 Methane0.6 Ethylene 4.41 Ethane 0.42 Propylene 37.0 Propane 3.82 C₅ ⁺ 53.35Total 100.00

[0065] TABLE 6 Composition in weight % of the C₅ ⁺ liquid fractionobtained with ZSM-12 (SiO₂/Al₂O₃ = 250 mol/mol) Reaction C₅ ⁺ T BTXNAPHTHALENES (non aromatics) OTHERS (° C.) % % % % 500 35 3.5 17.5 44

[0066] The term BTX refers to benzene, toluene and xylenes; the headingNAPHTHALENES comprises all hydrocarbons, variously substituted, of thenaphthaline family; the term C₅ ⁺(non aromatics) refers to non aromatichydrocarbons, saturated and mono-unsaturated, containing 5-8 carbonatoms. The term OTHERS comprises those products for which it was notpossible to effect a gaschromatographic characterization.

Comparative Example 5 Commercial ZSM-5

[0067] The catalytic testing of commercial ZSM-5 zeolite (CBV 3020 E)having a molar ratio SiO₂/Al₂O₃=30, was effected using the equipmentdescribed above and under the exact operating conditions described inexample 3.

[0068]FIG. 3 indicates the two curves relating to total conversion andselectivity to propylene, obtained with this zeolite in relation to thetime on stream (tos).

[0069] The conversion and selectivity to propylene are defined as inexample 3.

[0070] The catalytic performances of ZSM-5 are much lower both in termsof yield (product of selectivity and conversion) and duration, withrespect to the ZSM-12 zeolite.

[0071] It can be observed, in fact, from the graph of FIG. 3 how,already after 10 hours of tos, evident catalytic deterioration phenomenaare present.

[0072] Table 7 indicates, for illustrative purposes, the selectivity ofthe various components forming the product at the plant outlet.

[0073] Table 8 specifies the composition of the C₅ ⁺fraction subdividedby group of compounds. TABLE 7 Selectivity after 27 hours at aconversion of 85% Selectivity Product (weight %) Hydrogen 1.63 Methane3.70 Ethylene 3.03 Ethane 5.14 Propylene 3.84 Propane 34.15 C₅ ⁺ 48.51Total 100.00

[0074] TABLE 8 Composition in weight % of the C₅ ⁺ liquid fractionobtained with commercial ZSM-5 Reaction C₅ ⁺ T BTX NAPHTHALENES (nonaromatics) OTHERS (° C.) % % % % 500 80 — — 20

1. A process for the production of propylene starting from mixtures ofhydrocarbons, prevalently olefins, the above hydrocarbons having aboiling point ranging from −15° C. to +80° C., which comprises puttingthe above mixture of hydrocarbons in contact, under cracking conditions,with a large-pore zeolite having a molar ratio Silica/Alumina lower than200.
 2. The process according to claim 1, characterized in that themixture of hydrocarbons has a boiling point ranging from −12° C. to +60°C.
 3. The process according to claim 1, characterized in that thezeolite is a ZSM-12 zeolite.
 4. The process according to claim 3,characterized in that the ZSM-12 zeolite has a molar ratioSilica/Alumina ranging from 50 to
 150. 5. The process according to claim1, characterized in that the mixture of hydrocarbons comprises from 30%to 100% by weight of olefins.
 6. The process according to claim 5,characterized in that the mixture of hydrocarbons has a content of 40%to 85% by weight of olefins.
 7. The process according to claim 1,characterized in that the process is carried out at a temperatureranging from 400° C. to 750° C.
 8. The process according to claim 7,characterized in that the temperature ranges from 450° C. to 700° C. 9.The process according to claim 8, characterized in that the temperatureranges from 500° C. to 650° C.
 10. The process according to claim 1,characterized in that it is carried out at a weight hourly spacevelocity (WHSV) ranging from 0.1 h⁻¹ to 1,000 h⁻¹.
 11. The processaccording to claim 10, characterized in that the weight hourly spacevelocity ranges from 0.5 h⁻¹ to 100 h⁻¹.
 12. The process according toclaim 11, characterized in that the weight hourly space velocity rangesfrom 0.8 h⁻¹ to 50 h⁻¹.